Orthogonal Frequency Division Multiplexing (OFDM) refers to a data transmission scheme wherein a data stream is split into smaller sub-streams and transmitted using sub-carriers, each having a smaller bandwidth than the total available transmission bandwidth. The efficiency of OFDM results from the orthogonality of the sub-carriers. That is to say, the sub-carriers are selected such that they do not interfere with each other during transmission.
Multiple-Input Multiple-Output (MIMO) refers to a wireless transmission and reception scheme wherein communication devices employ multiple antennas to transmit and receive communication signals. A MIMO system takes advantage of the spatial diversity or spatial multiplexing options created by the presence of multiple antennas. In addition, a MIMO system improves signal quality, such as for example signal-to-noise ratio (SNR), and increases data throughput.
For practical reasons, OFDM may be preferred over other transmission schemes such as Code Division Multiple Access (CDMA), particularly in MIMO communication systems. When user data is split into sub-streams and carried by different sub-carriers, for example, the effective data rate on each sub-carrier is less than the total transmit data rate. As a result, the symbol duration of data transmitted with an OFDM scheme is much larger than the symbol duration of data transmitted with other schemes. Larger symbol durations are preferable as they can tolerate larger delay spreads. To illustrate, data transmitted with large symbol duration is typically less affected by multi-path than data transmitted with shorter symbol duration. Accordingly, OFDM symbols can overcome delay spreads that are common in wireless communications without the use of overly complicated receivers.
A continuing challenge for all types of wireless communications systems, including those employing OFDM- and CDMA-type transmission schemes, is the efficient use of available system resources, particularly during the transmission of data. In MIMO systems, this challenge becomes even more significant as multiple data streams are transmitted simultaneously.
A conventional approach utilized in CDMA communication systems to improve system efficiency is referred to as Per Stream Rate Control (PSRC). PSRC is a scheme particularly designed for use in CDMA systems whereby bandwidth, transmit power, and other resources are assigned to a transmitter based on channel conditions as seen by that transmitter. In CDMA systems having multiple transmitters and/or receivers, resources are similarly allocated on a per-transmit-antenna basis. There does not exist, however, a PSRC-type scheme for improving system efficiency in OFDM-MIMO communication systems.
Accordingly, it is desirable to have a method and apparatus for improving system efficiency in OFDM-MIMO communication systems.